Method and device for producing ball-shaped metallic particles at least almost equal in diameter

ABSTRACT

Method and device for producing ball-shaped metallic particles substantially equal in diameter are disclosed. The device comprises a cylindrical metallic housing and a vessel provided on the cylindrical metallic housing. The vessel has a plurality of small openings through a bottom plate thereof. A vibrator is disposed above the vessel in a manner that the vessel may be subjected to vibration. A pair of pipes are provided to deliver and fill nitrogen or inert gas within the housing. An inclined bottom plate having a soft layer is arranged at a bottom portion of the housing to form a shielding structure. An exhaust pipe is provided at an outlet of the inclined bottom plate, and a selector is arranged at a bottom outlet of the shielding structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

A great many of small ball-shaped metallic particles are widely used forsoldering or powder metallurgy. A number of methods and devices forproducing particles of this type from a molten metal are known. Forexample, particles of liquid metal are produced by means of dissectingliquid metal above a gas stream flowing against the force of gravity,wherein the particles adopt a ball shape due to the surface tension ofthe liquid metal and solidification.

In another method, a metallic wire or plate is cut into pieces, eachhaving a given length, which are pressed and rolled into small metallicparticles, or alternately the cut pieces are melted and the molten metalis rolled into small metallic particles.

The small ball-shaped metallic particles thus produced have thedisadvantage that there is a difference of the melting temperatures ofthe small metallic particles made from the starting material made froman end portion and that of a middle portion thereof.

As a result, there is a tendency that when spontaneous soldering ofsemiconductor circuits is carried out at a determined temperature, someportions of the circuits are not soldered.

It is likely that some metallic particles are either deformed or theirsurfaces are damaged.

A good small ball-shaped metallic particle usually has a brilliantmetallic luster on its surface so that the quality of the producedmetallic particle can be judged by quality of the luster of the smallball-shaped metallic particle as its standard.

2. Description of the Prior Art

According to the recent method described in the published specificationof Japanese Patent Publication No. 43794/1987 (not-examined), moltenmetal is sprayed into the air. As shown in the published specificationof Japanese Patent Publication No. 184201/1988, the method of producingthe small ball-shaped metallic particles is shown, in which a moltenstream of metal flows under pressure through an opening of a containerin order to produce the small ball-shaped metallic particles under thesurface tension.

There is a tendency, however, for the above method to produce a lot ofeither deformed or damaged small ball-shaped metallic particles, thusproviding a lower yield. Accordingly, it is rather difficult toefficiently produce a number of good small ball-shaped metallicparticles.

As shown in the published specification of Japanese Patent PublicationNo. 144216/2000 (not-examined), liquid metal is discharged through anopening of a container to atomize in the air and to solidify intodroplets by the surface tension of the liquid metal.

In accordance with this method, however, liquid metal particles aredropped into an oily solution which does not vaporize even at 200° C. inorder to prevent collision of the dropping small particles with eachother or to avoid a deformation by a shock on a hard bottom plate.

This method, however, has the disadvantage that the metallic surfaces ofthe produced metallic particles are oxidized by the oily solution,thereby preventing an independent brilliant metallic luster on thesurfaces of the produced small ball-shaped metallic particles.

SUMMARY OF THE INVENTION

A primary object of this invention is to provide a method for producingsmall ball-shaped metallic particles substantially equal in diameterthat can be used for soldering a base circuit of semiconductors.

Another object of this invention is to provide a device for producingsmall ball-shaped metallic particles substantially equal in diameterthat can be used for soldering a base circuit of semiconductors.

Another object of this invention is to provide a device for producingsmall ball-shaped metallic particles substantially equal in diameterhaving better physical, structural and homogeneous properties.

Another object of this invention is to provide a method for producingsmall ball-shaped metallic particles substantially equal in diameterwithout a troublesome washing process for recovering the metallicparticles in an oily solution.

Another object of this invention is to provide a method for producingsmall ball-shaped metallic particles substantially equal in diameterwithout a troublesome disposal of waste oil.

Another object of this invention is to provide an easily made device,which is simple in construction.

Another object of this invention is to provide a method for producingsmall ball-shaped metallic particles substantially equal in diameterwhereby soft droplets drop on a soft inclined bottom plate, thuseliminating dents, flaws or deformations on the surface of the metallicparticles as a result of cushioning of the soft inclined bottom plate.

Still another object of this invention is to provide small ball-shapedmetallic particles substantially equal in diameter.

BRIEF DESCRIPTION OF THE DRAWINGS

This object and features of the invention will become apparent from thefollowing description taken in conjunction with the preferred embodimentthereof with reference to the accompanying drawings, in which:

FIG. 1 is a front elevation, partly in perspective, of a device forproducing ball-shaped metallic particles at least almost equal indiameter;

FIG. 2 is a plan view of the device shown in FIG. 1;

FIG. 3 is an enlarged schematic longitudinal sectional view of a devicefor producing small ball-shaped metallic particles at least almost equalin diameter of this invention, taken on line 111—111 of FIG. 1; and

FIG. 4 is a flow-chart showing the steps of a method for producing smallball-shaped metallic particles at least almost equal in diameter of thisinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, especially FIGS. 1-3, a device 10 forproducing small ball-shaped metallic particles substantially equal indiameter comprises a cylindrical metallic housing 12 and a vessel 14which is provided through a central opening 12 b located at a middleportion of the ceiling 12 a of the cylindrical metallic housing 12.

Provided through a bottom plate 14 a of the vessel 14 are a plurality ofsmall openings 14 b which are so small that a supply of liquid or moltenmetal 30 does not flow downwardly at a standstill condition by thesurface tension of the liquid or molten metal 30.

A vibrator 18 is disposed on a mount 16, which is arranged on theceiling 12 a to locate above the vessel 14 in a manner that the vessel14 may be subjected to vibration by the vibrator 18.

A pair of pipes 24, each having a valve 24 a, is provided to locate at amiddle of opposing sides of the cylindrical metallic housing 12, thusenabling to deliver and fill nitrogen or an inert gas G within thehousing 12.

An inclined bottom plate 20 having a soft layer 20 a is arranged at abottom portion of the housing 12 to form a shielding structure 22. Anexhaust pipe 26 is provided outwardly at an outlet of the inclinedbottom plate 20.

A selector 26 a such as mesh, gravitation or wind selecting means isarranged at a bottom outlet 20 d of the exhaust pipe 26.

In another embodiment, either soft plate 20 a or cloth 20 b, each havinga high igniting point, which does not burn even at a high temperaturecan be mounted on the inclined bottom plate 20.

For example, an inorganic fiber such as rock wool, vegetable fiber or asoft plate made from pulp can be used for the soft plate 20 a.

Inorganic fiber that is made from a woven fabric or soft cloth ofvegetable fiber is used for the soft cloth 20 b.

A composite or a plurality layer of the soft plates 20 a or the softcloth 20 b can be preferably used, or alternatively a composite layer ofthe soft cloth 20 b having a hollow air layer can be used.

It should be understood that a number of approximately small ball-shapedmetallic particles 3 d almost equal in diameter and having a luster oneach surface could be manufactured continuously by the method and deviceof this invention.

EXAMPLE 1

The liquid or molten metal 30 is supplied into the vessel 14 having aplurality of the small openings 14 b that are so small that the liquidor molten metal 30 is not allowed to flow downwardly through theopenings 14 b at a standstill condition as a result of the surfacetension of the liquid metal 30.

A diameter of the opening 14 b of the bottom plate 14 a is small enoughto prevent the liquid or molten metal 30 from flowing downward throughthe openings 14 b.

When the vessel 14 is subjected to vibration by the vibrator 18, theliquid or molten metal 30 flows downwardly through the small openings 14b so that the segments of the dissected stream take a ball shape due tothe surface tension of the liquid or molten metal 30 into a number ofthe molten droplets 30 a in the shielding structure 22.

Further, the molten droplets 30 a take on a ball shape during a freefall through the nitrogen or inert gas G surrounding them in theshielding structure 22.

The molten droplets 30 b are cooled and solidified while dropping. Thesolidified droplets 30 c drop and roll on the soft plate 20 a or thesoft cloth 20 b towards the outside of the inclined bottom plate 20.

The solidified spherical droplets 30 c rolling down on the soft plate 20a or the soft cloth 20 b are delivered into a container 28, which issubsequently subjected to a selection by a known selecting device (notshown) to select a number of small approximately ball-shaped metallicparticles 30 d almost equal in diameter and having a luster on eachsurface of the metallic particles 30 d.

EXAMPLE 2

The soft plate 20 a or the soft cloth 20 b, each having a high ignitingpoint, is preferably used for the soft inclined bottom plate 20.

In accordance with the method and device of this invention, smallapproximately ball-shaped metallic particles 30 almost equal in diameterand having a luster on each surface can be manufactured continuouslywith a high yield.

EXAMPLE 3

A method for producing small ball-shaped metallic particlessubstantially equal in diameter of this invention is explained withreference to an embodiment shown in FIGS. 3 and 4.

As in the foregoing embodiments, a plurality of small openings 14 b isprovided at a given distance from the inclined bottom plate 20. Theopenings 14 b are small enough so that the molten droplets 30 a cannotflow through the opening 14 b of the bottom plate 14 a at a staticcondition.

A diameter of the opening 14 b can be determined based on the type ofmetal used as the liquid metal 30, such as, tin, bismuth, zinc, aluminumetc.

When a diameter of the opening 14 b of the bottom plate 14 a is lessthan 0.2 mm, liquid metal 30 does not flow through the openings 14 b byvibration only. Accordingly, it is necessary to apply additionalpressure.

In this embodiment, the vessel 14 is subjected to vibration by thevibrator 18 to deliver the liquid metal 30 downward through the smallopenings 14 b into an atmosphere containing no oxygen or inert gas Gcomprising nitrogen or a mixture of nitrogen and another gas in theshielding structure 22.

While dropping, the molten droplets 30 a take on a ball shape while freefalling through the nitrogen or inert gas G surrounding them in theshielding structure 22 as a result of the surface tension. In thisembodiment, no oxygen exists around the surface of each droplet 30 a sothat it is not oxidized to have a metallic luster on the surface of thedroplet 30 a.

While dropping further, the ball-shaped droplets 30 a are cooled into acertain degree and solidified.

Solidification of the dropping ball-shaped droplets 30 a depends on thetemperature of the atmosphere in the housing 12. Thus solidificationrequires a long cooling time along for a melting temperature and a givendropping height of the ball-shaped droplets 30 a.

For example, a soldering material is dissolved at a temperature between183° C.˜300° C., and the dissolved soldering material solidifies at roomtemperature at a height between 5 m˜6 m so that a height for droppingthe metallic particles 30 a must be over 5 m˜6 m.

Aluminum is dissolved at a temperature around 600° C., and the dissolvedaluminum solidifies at room temperature at a height between 45 m˜50 m sothat a height for dropping the metallic particles must be over 50 m.

When liquefied nitrogen is used to make a nitrogen atmosphere in theshielding structure 22, the liquefied nitrogen itself is used as acooling gas at vaporization.

When the dropping atmosphere can be cooled, a cooling solidification canbe accelerated to shorten the dropping height.

The solidified small ball-shaped metallic particles 30 c drop on thesoft inclined bottom plate 20, on which shock of the dropped ball-shapedmetallic particles 30 c are softened and dispersed by a cushion in orderto prevent dents and deformation of the surface of the ball-shapedmetallic particles 30 c.

When the ball-shaped metallic particles 30 c directly and vertically hitthe hard inclined bottom plate 20, some of the ball-shaped metallicparticles 30 c are crushed or some plane portions thereof are formed onthe spherical surfaces 30 c. This destroys the small ball-shapedmetallic particles substantially equal in diameter and also loses thebrilliant metallic luster on the surfaces of the ball-shaped metallicparticles 30 c.

Subsequently, the ball-shaped metallic particles 30 c roll downward intothe exhaust pipe 26.

As mentioned in the foregoing paragraphs, when the ball-shaped metallicparticles 30 c directly and vertically hit the hard inclined bottomplate 20, the ball-shaped metallic particles 30 c are either deformed orhurt by the dropping shock to lose brilliant metallic luster on thesurfaces of the ball-shaped metallic particles 30 c.

The ball-shaped metallic particles 30 c are delivered into the selectingdevice to select a number of small approximately ball-shaped metallicparticles 30 d almost equal in diameter and having a luster on eachsurface.

EXPERIMENTAL EXAMPLE 1

The ball-shaped metallic particles 30 d having a mixture of 63% of tinand 37% of lead were dropped on the hard inclined bottom plate 20through the opening 14 b of the bottom plate 14 a from a height of 4 m.

In case a diameter of the opening 14 b of the bottom plate 14 a is lessthan 0.65 mm, the small ball-shaped metallic particles 30 d almost equalin diameter and having no flaw, but having the luster on each surfacecan be made with a high yield.

In case a diameter of the small ball-shaped metallic particle 30 d isover 0.66 mm, there appear many flaws and few luster on each surface ofthe produced small ball-shaped metallic particle 30 d.

It has been confirmed by this experiment that collision of the smallball-shaped metallic particles 30 d deteriorate their quality, and thatthe smaller the diameter of the particle 30 d, the fewer the poorparticles 30 d.

EXPERIMENTAL EXAMPLE 2

The ball-shaped metallic particles 30 d having a mixture of 10% of tinand 90% of lead were dropped on the hard inclined bottom plate 20through the opening 14 b of the bottom plate 14 a from a height of 4 m.

In case a diameter of the small ball-shaped metallic particle 30 d is0.8 mm, the solidified ball-shaped metallic particle 30 d is somewhatsoft and there appear flaws on a surface of the 100% particles 30 d.

When the soft solidified small ball-shaped metallic particles 30 c dropon the soft inclined bottom plate 20, there appear no dents, flaws ordeformation on the surface of the metallic particles 30 c by a cushionof the soft inclined bottom plate 20.

This invention provides the method and device for producing smallball-shaped metallic particles substantially equal in diameter.

It is understood that modifications to the invention as described may bemade, as might occur to one with skill in the field of the invention,within the scoped of the appended claims. All embodiments contemplatedhereunder, which achieve the objects of the invention, have thereforenot been shown in complete detail. Other embodiments may be developedwithout departing from the spirit of the invention or from the scope ofthe appended claims.

I claim:
 1. A method of producing ball-shaped particles, said methodcomprising: supplying liquid into a vessel comprising a bottom platehaving a plurality of openings therein; vibrating the vessel therebydelivering streams of the liquid down through the plurality of openings;passing the streams of the liquid through a sufficient amount of gassuch that the gas dissects the streams thereby forming a plurality ofdroplets, such that the gas affects a surface tension of the dropletspassing therethrough thereby creating ball-shaped particles and suchthat the gas solidifies the ball-shaped particles; and positioning abottom plate such that the solidified ball-shaped particles impact thebottom plate at an angle that is not normal to the bottom plate and movealong a length of the bottom plate.
 2. The method of claim 1, furthercomprising: delivering the solidified ball-shaped particles into acontainer; and selecting, with a selecting device, a number of thesolidified ball-shaped particles that are substantially equal indiameter and the have a luster on an outer surface thereof.
 3. Themethod of claim 2, wherein said supplying liquid comprises supplyingliquid metal.
 4. A device for producing ball-shaped particles, saiddevice comprising: a housing having a ceiling and a bottom, said ceilinghaving a ceiling opening, said bottom having a bottom opening; a vesselcomprising a bottom plate having a plurality of openings and beinglocated at said ceiling opening, each of said plurality of openingshaving a size that prevents a liquid from flowing therethrough whileunder a static condition as a result of a surface tension of the liquid,a mount disposed on said ceiling of said vessel, a vibrator disposed onsaid mount in such a manner as to be operable to vibrate said vesselsufficiently to permit streams of the liquid to flow through saidplurality of openings toward said bottom of said housing; a pipedisposed in communication with said housing and operable to deliver gasinto said housing; and a plate located a direction of flow each of thestreams of liquid flowing through said plurality of openings toward saidbottom of said housing and disposed such that the direction of flow isnot normal to said plate.
 5. The device of claim 4, further comprisingan exhaust pipe in communication with said bottom opening, wherein saidplate is disposed at a position adjacent to said exhaust pipe and at alocation sufficiently separated from said ceiling of said housing suchthat each of the streams of liquid that flows through one of saidplurality of openings toward said plate separates into a plurality ofdroplets that solidify prior to contacting said plate, and wherein saidplate is disposed at an angle that enables solidified droplets to movetoward said exhaust pipe.
 6. The device of claim 5, further comprising aselector that is capable of permitting solidified droplets of asubstantially predetermined size and shape to pass through said exhaustpipe.
 7. The device of claim 6, wherein said selector comprises a mesh,a gravitation selector or a wind selector.
 8. The device of claim 7,wherein said selector is disposed approximate to said bottom opening ofsaid housing.
 9. The device of claim 8, further comprising a valvedisposed in said pipe and being operable to control an amount of gas tobe delivered into said housing.
 10. The device of claim 9, furthercomprising: a second pipe disposed in communication with said housingand operable to deliver gas into said housing, and a second valvedisposed in said second pipe and being operable to control an amount ofgas to be delivered into said housing.
 11. The device of claim 10,wherein said pipe and said second pipe are capable of deliveringnitrogen or an inert gas into said housing.
 12. The device of claim 11,wherein said housing comprises a cylindrical metallic housing.
 13. Thedevice of claim 12, wherein each of said plurality of openings have asize that prevents a stream of liquid metal from flowing therethroughwhile under a static condition as a result of a surface tension of theliquid metal.
 14. The device of claim 13, wherein said plate is disposedat a location sufficiently separated from said ceiling of said housingsuch that each of the streams of liquid metal that flows through one ofsaid plurality of openings toward said plate separates into a pluralityof metal droplets and solidifies prior to contacting said plate, andwherein said plate is disposed at an angle that enables solidified metaldroplets to move toward said exhaust pipe.
 15. The device of claim 14,wherein said plate comprises a material layer disposed such that thesolidified metal droplets contact said material layer, and wherein saidmaterial layer is softer than that of a portion of said plate that isdisposed beneath said material layer.
 16. The device of claim 15,wherein said material layer comprises at least one of a plate or cloth.17. The device of claim 16, wherein said material layer comprises atleast one of inorganic fiber, vegetable fiber and pulp.
 18. The deviceof claim 17, wherein said inorganic fiber comprises rock wool.
 19. Thedevice of claim 18, wherein said inorganic fiber comprises woven fabric.20. The device of claim 19, wherein said material layer comprises aplurality of layers.
 21. The device of claim 20, wherein said pluralityof layers comprises at least one layer of air.
 22. The device of claim21, wherein said cylindrical metallic housing contains inert gascomprising nitrogen.
 23. The device of claim 4, wherein each of saidplurality of openings have a size that prevents a stream of liquid metalfrom flowing therethrough while under a static condition as a result ofa surface tension of the liquid metal.
 24. The device of claim 23,wherein said plate is disposed at a location sufficiently separated fromsaid ceiling of said housing such that each of the streams of liquidmetal that flows through one of said plurality of openings toward saidplate separates into a plurality of metal droplets and solidifies priorto contacting said plate.